The terms oxymethylene polymer and polyoxymethylene as used interchangeably herein are meant to include oxymethylene homopolymers and diethers and diesters. Also included are oxymethylene copolymers, which includes oxymethylene polymers having at least 60 percent recurring oxymethylene units and at least one other unit derived from a monomer copolymerizable with the source of the oxymethylene units.
Oxymethylene polymers having recurring --CH.sub.2 O--units have been known for many years. They may be prepared for example, by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde, and will vary in physical properties such as thermal stability, molecular weight, molding characteristics, color and the like depending, in part, upon their method of preparation, on the catalytic polymerization technique employed and upon the various types of comonomers which may be incorporated into the polymer.
During molding processes, oxymethylene polymers are generally heated to temperatures between about 180.degree. and 220.degree. C. for relatively short periods between about 3 and about 10 minutes. Unless the thermal degradation rate of the polymer has been reduced to a desirably low level it has been found that molded products have uneven surface characteristics, due to gassing. In order to ameliorate this defect and reduce the degradation to a desirable level, chemical stabilizers are added to oxymethylene polymers.
The physical properties of these oxymethylene polymers can also be enhanced by addition to the polymers of glass strands, such as chopped glass fibers. In so doing, the tensile strength, flex strength and modulus are increased but the thermal coefficient of expansion of the polymer is reduced and warpage is significantly increased. Unfortunately, addition of these glass fibers also has had an undesirable effect on thermal stability and impact strength, thus limiting the use, to a certain degree, to which the thus-filled oxymethylene polymers might be put. Another drawback is that the molded reinforced oxymethylene polymers often exhibit rough surfaces because the reinforcing agents tend to extend through the polymer surface.
It is known in the art that the incorporation of glass strands into oxymethylene polymer in the presence of a small but effective amount of a halogen acid significantly improves the physical properties of these glass-filled oxymethylene polymers. The preferred halogen acid-yielding additives contemplated herein are the ammonium and amine salts of hydrogen chloride, hydrogen bromide and hydrogen iodide. Very desirable results are also achieved by using the acids per se, aluminum chloride plus water, polyvinyl chloride, and like halogen acid (HX)-yielding compounds. Generally, from 0.001% by weight to 0.1%, or slightly greater, of the acid is present for good results, the percentage by weight being based upon the total weight of the polymer. Preferably, from 0.005% to 0.02% by weight is employed. The glass strands, on the other hand, may be present, and preferably are present, in substantially greater amounts. For example, good results may be achieved when blending equal weights of oxypolymer and glass strands. Enhanced properties are achieved using as little as 10 weight percent glass strands based upon the total weight of the glass and polymer in the composition. This is particularly true when using chopped glass fibers in the one-sixteenth (1/16") inch to one-half (1/2") inch size range.
It is also known in the art that incorporation of both an isocyanate and glass strands in oxymethylene polymers, preferably in the presence of a small but effective amount of catalyst, has a potentiating or synergistic effect on the physical properties of these polymers (U.S. Pat. No. 3,455,867). For reasons which are not fully understood, not only are tensile strength and flex strength improved, but impact strength is enhanced. Insofar as tensile strength and flex strength are concerned, the cooperative effect of the isocyanate and glass strands in the polymer is such that the total effect of these is greater than the sum of the two effects taken independently. This phenomenon is even more accentuated by the presence of the catalyst. However, a chemical reaction is required and, thus, it is difficult to control this in-situ reaction to obtain a reproducible product.
Another filled oxymethylene polymer composition known in the art is disclosed in U.S. Pat. No. 3,901,846 and includes a filler and small amounts of specific high molecular weight phenoxy resins and exhibits improved physical properties and surface effects of the molded articles. It is disclosed that the type of filler that can be used includes glass fibers (chopped or continuous rovings), asbestos fibers, cellulosic fibers, synthetic fibers, including graphite fibers, acicular calcium metasilicate and the like. The amount of reinforcing agent can range from about 2 to about 60 weight percent, preferably 5 to 60 weight percent based on the total molding composition. The phenoxy resins disclosed as useful are characterized by a repeating structure: ##STR1## and have an average molecular weight range of from about 15,000 to 75,000. As is obvious, the terminal structure is completed with hydrogen atoms or some suitable end capping groups. The thermoplastic phenoxy resin can be added to the reinforced oxymethylene resin in a number of ways (1) by incorporating the phenoxy resin into the reinforcing agent prior to its intimate blending with the oxymethylene resins, (2) by simultaneously intimately mixing with the reinforcing agent and the oxymethylene resins and (3) by blending with the polymer and intimately blending with the reinforcing agents. Other mixing techniques can be used. The amount of phenoxy resin incorporated can range from about 0.1 to about 8 weight percent preferably from about 0.5 to about 3 weight percent of the total thermoplastic oxymethylene molding resin.
Commonly assigned U.S. Pat. No. 4,613,634 discloses a low warp, filled oxymethylene polymer composition containing 1 to about 60 wt. % glass beads having an average bead diameter distribution of from greater than 0 up to 300 microns, and a thermoplastic phenoxy resin as heretofore described. As compared with the glass fiber compositions of U.S. Pat. No. 3,901,846, the glass bead-filled oxymethylene polymer compositions have greatly reduced warpage.
While reinforced polyoxymethylene polymers are outstanding thermoplastic molding resins and each of the above-described prior art compositions is useful, it is still desired to improve the combination of surface appearance flexural modulus, tensile strength, and low warpage in the products molded from these resin composites which some applications require. The present invention provides such a composition.